How to measure the coastal currents of Surat?

1. Where is Surat?

Surat is a bustling city in the state of Gujarat, India, situated near the Arabian Sea on the banks of the Tapi River. It is an important economic center and is mainly known for its diamond and textile industries.

The cityscape of Surat is a perfect blend of modern infrastructure coupled with historical architecture. In addition, the local culture is rich and varied because a wide range of festivals and traditions are celebrated across the year. People of Surat are engaged in almost all types of occupations; however, the majority falls under the commercial and industrial sector.

Regarding its coastal connectivity, the Tapi River falls into the Arabian Sea near Surat. This forms an estuarine environment with high levels of nutrients and hence very varied marine life. Most species of fish, crustaceans, and mollusks are found within these adjoining coastal waters. The Arabian Sea is used as a very active trade route, and its port at Surat receives quite a lot of cargo traffic. The coastline near Surat also features sandy beaches and tidal flats, which are influenced by the ebb and flow of the tides.

2. What is the situation of the coastal currents near Surat?

Various factors are involved in the coastal currents near Surat. Due to the proximity of the estuary, tidal currents dominate. The gravitational pull of the moon and the sun causes the tides, and the associated tidal currents can be quite strong. The mixing of the river water and seawater in the estuary also affects the density - driven currents.

The most significant influence is the monsoon winds, mainly the southwest monsoon between June and September. The surface waters are driven by these winds, which changes the current pattern. Seaward shape of the coastline and bottom topography around Surat have their role in the currents. The shoals, channels, and the mouth of the Tapi River can deflect and accelerate the flow of water.

Furthermore, the Arabian Sea's large-scale ocean circulation patterns may interact with the local coastal currents. Inflow of different water masses from other regions can bring changes in temperature, salinity, and overall current characteristics.

3. How to observe the coastal water flow of Surat?

Surface Drift Buoy Method: Buoy deployment on the surface of the water, whose drift is allowed by the currents. The movement of buoys, tracked from satellites or with other locationing methods, will eventually give us data on speed and direction of the superficial currents. In general, this method represents the surface layer and usually does not provide data relevant to the current at different depth levels.

Anchor Boat Method: In this method, a boat is anchored in the coastal waters. Instruments are then used to measure the water flow around the boat at various depths. While this can provide more detailed data for the area around the boat, it is a relatively labor-intensive and time-consuming method. It also has limitations as to the area it can cover.

Acoustic Doppler Current Profiler (ADCP) Method: The ADCP current meter is a more modern and time-saving means of measuring coastal currents. It applies sound waves to measure the water's velocity at many depths all at once. It provides a detailed profile of the current from the surface down to a certain depth, which gives an accurate and comprehensive picture of the coastal currents compared to other methods.

4. How do ADCPs based on the Doppler principle work?

The working principle of an ADCP current profiler is based on the Doppler effect. ADCPs emit sound pulses into the water. In their interaction with particles in water-sediment particles, plankton, or any other small organisms-the frequency of the reflected sound waves changes owing to the Doppler effect. If the particles are moving towards the ADCP meter, the frequency of the reflected waves is higher than the emitted waves, and if they are moving away, the frequency is lower.

By measuring the Doppler shifts at different reflective angles and depth, ADCP flow meter can determine the magnitude of water velocity in several directions. ADCPs, with multiple transducers working in a ‘ping-and-listen' mode, will be able to develop a rather detailed profile of water velocity from the surface all the way down through the water column to some depth restriction, depending on the capabilities of the instrument.

5. What's needed for high-quality measurement of Surat coastal currents?

The equipment required to measure the coastal currents at Surat should be of a nature that has been constructed with reliable materials, since the marine environment in which it will be located is very harsh, with salt-water corrosion, temperature fluctuations, and the possibility of physical impacts from floating debris.

The size of the equipment shall be as small as possible to easily deploy and install. No matter whether on boats, buoys, or at the seabed, a small size makes the handling and installation easier, and so does the transportation and maintenance more convenient.

The weight of the equipment should be light, especially for floating or moored devices. A light weight ensures proper buoyancy and stability. Low power consumption is also very important because it allows for continuous operation over long periods without the need for frequent battery replacements or a large power supply.

Cost-effectiveness is another issue that has to be seriously considered. In order for the large-scale measurements in a coastal area, the measuring device should not be unaffordable. In general, for ADCPs a casing made of titanium alloy would be an excellent option. The corrosion resistance of titanium alloy is great, which is just crucial in the salty sea environment. It is also durable and relatively light, fit for the requirements of being a high-quality current-measurement equipment.

6. How to Select appropriate equipment for current measurement?

While selecting equipment for current measurement at a location near Surat, the purpose of usage has to be considered first.

Ship-borne ADCP: It is suitable for measurement from a moving vessel. It records the coastal currents continuously as the ship moves along the coast, providing a series of data points over a relatively wide area covered by the ship's route.

Bottom-mounted ADCP: Suitable for fixed-point measurements at the seabed. It provides a stable and long-term record of the water flow near the bottom, which is important for understanding the interaction between the bottom topography and the coastal currents.

Buoy-mounted ADCP: This is used where one wants to measure currents within a particular area for long periods while floating at the water surface. These instruments can be deployed where interesting areas are and can continuously make measurements without the need for vessels.

According to frequency, for the water depth within 70m, 600kHz ADCP should be chosen. For depth up to 110m, it is more suitable to use a 300kHz ADCP. And for more deep waters, up to 1000m, a 75kHz ADCP would be recommended.

There are well-known ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, a Chinese brand, China Sonar PandaADCP, is also worth mentioning. It has a casing made entirely of titanium alloy and offers an excellent cost-performance ratio. You can find more information about it at https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and moels.